Silicon is not found in nature as the free element but instead occurs in various minerals, such as silica and silicates, amounting to approximately one-fourth of the earth's crust. Compounds containing silicon account for its greatest usage, such as silica in glass manufacture. However, elemental silicon itself is commercially important with very important usages thereof being in semiconductors, transistors, rectifiers, and like electronic components. Additional usages of elemental silicon are in metal alloy preparation, for deoxidizing steel, in providing protective coatings, and in thermal energy applications such as a solar thermal absorber and such as in photovoltaic cells for direct conversion of solar energy to electricity through their absorption of incident solar photons. Although used as the element per se in some such applications, more generally the elemental silicon is accompanied, i.e. doped, by controlled, minute amounts of other materials to provide particular properties requisite for its specific utility.
The elemental silicon purity required for a particular usage generally dictates its particular method of preparation. An ordinary commercially pure form of silicon, such as for an alloying additive, can be obtained by reduction of silicon dioxide with carbon or calcium carbide in an electric furnace. However, where exceedingly high purity silicon is required, more complex and lengthy procedures are followed. For example, the ordinary chemically pure silicon, such as from an electric furnace preparation, is converted to a silicon halide or haloid silane, which then by fractional distillation is subsequently purified (e.g. freed of B, As, etc.). Such purified silicon compound then may be reconverted to elemental silicon of high purity, such as by hydrogen reduction in a hot tube or on a hot wire for silicon tetrachloride and silicon tetrabromide and such as by direct thermal decomposition on a hot wire for silicon tetraiodide. Thermal decomposition of silanes is another preparation route. To proceed to a "super-pure" silicon, such as requisite for electronic applications, the "pure" silicon generally is subjected to molten zone refining or the like. Illustrating the preceding are teachings in U.S. Pat. Nos. 2,944,874 (silicon by hot wire decomposition of liquid silanes), 3,011,877 (silicon by thermal decomposition or the like of gaseous silicon compounds), 3,014,791, (silicon by pyrolysis of silanes), 3,029,135 (purifying gases used in producing silicon), 2,747,971 and 2,901,325 (molten zone refining of silicon), and West German Patent No. 1,071,680 (silicon by reduction of silicon halide in organic solvents by alkali and alkali earth metals).
The prior art includes teachings of silicon electrodeposition from fused and/or molten salts or the like, such as illustrated in U.S. Pat. No. 3,022,233 and J. Can. Met. Quart. (1971), Nos. 4, p. 281-5. Such electrolyses in molten materials require high temperature; for example, greater than 1,000.degree. C in the process taught in the just-mentioned Canadian journal. The high temperature introduces problems of containing the molten bath and the formation of silicides. Also, there is considerable opportunity for impurities to diffuse into the silicon. Only a brief note is known of a possible electrolysis of silicon at a much lower temperature, which note reads
"As regards the action of silicon on metallic mercury, nothing very definite can at present be stated; but on subjecting a small vessel containing mercury in contact with an alcoholic solution of silicon fluoride to the action of a powerful battery, and afterwards subjecting the mercury to distillation, a small amount of amorphous silicon was obtained, but whether silicon, when in a nascent state combines with, or is soluble in, mercury still presents considerable doubt." H. N. Warren, "The Action of Silicon On The Metals Gold, Silver, Platinum, and Mercury," Chemical News, June 30, 1893, p. 303-4.
Metallic germanium, an element in Group IV of the Periodic Table, is taught as being electrodeposited from a solution of germanium tetrachloride in propylene glycol (G. Szekely, "Electrodeposition of Germanium," J. Electro. Chem. Soc. 98 (1951) p. 318-324). Also the art includes teachings of electroplating baths containing dimethyl sulfoxide (U.S. Pat. No. 3,616,280), tetrahydrofuran (U.S. Pat. No. 3,595,760), and propylene carbonate (U.S. Pat. No. 3,580,828) for electrodepositing various metals.
The present invention's electroplating composition and method of electrolytic deposition of elemental silicon provides and/or promises numerous important advantages over prior art of knowledge to the inventor. For example, the invention's method is operable at and/or near convenient room temperature and without employment of pressures differing greatly from atmospheric, even though a dry and inert gaseous atmospheric and substantially anhydrous and oxygen-free materials are employed. An important advantage of the invention is its providing low cost elemental silicon at an expected cost of deposited silicon at least in the ballpark of a several orders of magnitude less than that of presently available silicon for solar thermal absorber applications and with greater savings realized in comparison to conventional ultra-pure and doped ultra-pure silicon for photoelectric solar cell applications. Another important advantage of the invention is a decreased energy requirement of at least one order of magnitude in comparison with present processes for producing high purity silicon.